(a) Field of Invention
This invention relates to lead alloys having a low antimony content and containing arsenic, tin and copper. The alloys have utility in high capacity, maintenance-free battery grids.
(b) State of the Art
Lead-antimony alloys have been used as grid materials for lead acid batteries. Antimony is used to increase the strength and/or other physical properties of lead, thereby facilitating various aspects of battery manufacture. In the case of lead-acid battery grids, this is particularly important in order for the grids to withstand normal handling during battery manufacturing and service.
The battery industry has begun producing batteries which require little or no maintenance, such as addition of water to maintain the electrolyte level during the service life of a battery. In such batteries it is the practice to either seal the battery or use vent plugs for the filling ports which are not easily removed by the ultimate battery user. Since the purpose of such batteries is to eliminate the need for filling, a lead alloy system must be selected in which the supply of electrolyte will not be significantly diminished over the intended life of the battery. The presence of antimony typically causes excessive gas generation in lead-acid batteries, especially during the periods of charging or overcharging, which ultimately depletes the quantity of electrolyte. Such gassing is unacceptable in reduced or no-maintenance batteries particularly if they are of the completely sealed type.
Alloys containing no antimony, such as lead-calcium-tin, lead-strontium-tin-aluminum, and lead-calcium-tin-aluminum alloys, have been introduced as maintenance-free battery grid alloys to meet the requirements of cold cranking performance of the batteries. Lead-antimony alloys having above 2.5% antimony are not adequate for high capacity, maintenance-free battery grid alloys; rather, the antimony content must be further reduced to reduce water loss or gassing in batteries during charging and increase the conductivity of the grid alloy, thus increasing the cold cranking performance of the battery. However, elimination of antimony from the battery can result in formation of nonconducting layers at the grid-active material interface, thereby reducing battery performance.
According to the lead-antimony phase diagram, the freezing range becomes a maximum at about 3.5% antimony and antimony alloys containing less than 3.5% antimony should have reduced freezing range and no eutectic liquid. In fact, the amount of eutectic liquid is greatly reduced. However, because of segregation effects during solidification, some eutectic may be present in alloys of 1% antimony or less, indicating that the freezing range, instead of becoming narrower, becomes wider as the antimony content is decreased. The combination of increased freezing range and reduced eutectic liquid makes alloys in the 1-2% antimony content range very difficult to cast without cracking. To permit the use of alloys in this range, resort has been had to addition of nucleants, such as selenium, sulphur, copper, phosphorous, or aluminum, to prevent cracking. In these alloys, problems of temperature control, loss of nucleants and adverse reactions may occur and lead to loss of the alloying elements in use and produce cracking.
It has now been discovered that by restricting the antimony content of alloys to less than 1.1%, both the freezing range and amount of eutectic material are reduced. However, where such alloys are cast as battery grids, at the grid intersections or points where there are large differences in cross section which can cause solidification at different rates, some cracking can still occur due to concentration of eutectic liquid. It has further been discovered that such cracking can be eliminated by the addition of copper to the alloy. The low antimony alloys of the invention are suitable for use as battery grids in maintenance-free, high capacity batteries.